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//===- JumpTableToSwitch.cpp ----------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/JumpTableToSwitch.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
static cl::opt<unsigned>
JumpTableSizeThreshold("jump-table-to-switch-size-threshold", cl::Hidden,
cl::desc("Only split jump tables with size less or "
"equal than JumpTableSizeThreshold."),
cl::init(10));
// TODO: Consider adding a cost model for profitability analysis of this
// transformation. Currently we replace a jump table with a switch if all the
// functions in the jump table are smaller than the provided threshold.
static cl::opt<unsigned> FunctionSizeThreshold(
"jump-table-to-switch-function-size-threshold", cl::Hidden,
cl::desc("Only split jump tables containing functions whose sizes are less "
"or equal than this threshold."),
cl::init(50));
#define DEBUG_TYPE "jump-table-to-switch"
namespace {
struct JumpTableTy {
Value *Index;
SmallVector<Function *, 10> Funcs;
};
} // anonymous namespace
static std::optional<JumpTableTy> parseJumpTable(GetElementPtrInst *GEP,
PointerType *PtrTy) {
Constant *Ptr = dyn_cast<Constant>(GEP->getPointerOperand());
if (!Ptr)
return std::nullopt;
GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr);
if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer())
return std::nullopt;
Function &F = *GEP->getParent()->getParent();
const DataLayout &DL = F.getDataLayout();
const unsigned BitWidth =
DL.getIndexSizeInBits(GEP->getPointerAddressSpace());
MapVector<Value *, APInt> VariableOffsets;
APInt ConstantOffset(BitWidth, 0);
if (!GEP->collectOffset(DL, BitWidth, VariableOffsets, ConstantOffset))
return std::nullopt;
if (VariableOffsets.size() != 1)
return std::nullopt;
// TODO: consider supporting more general patterns
if (!ConstantOffset.isZero())
return std::nullopt;
APInt StrideBytes = VariableOffsets.front().second;
const uint64_t JumpTableSizeBytes = DL.getTypeAllocSize(GV->getValueType());
if (JumpTableSizeBytes % StrideBytes.getZExtValue() != 0)
return std::nullopt;
const uint64_t N = JumpTableSizeBytes / StrideBytes.getZExtValue();
if (N > JumpTableSizeThreshold)
return std::nullopt;
JumpTableTy JumpTable;
JumpTable.Index = VariableOffsets.front().first;
JumpTable.Funcs.reserve(N);
for (uint64_t Index = 0; Index < N; ++Index) {
// ConstantOffset is zero.
APInt Offset = Index * StrideBytes;
Constant *C =
ConstantFoldLoadFromConst(GV->getInitializer(), PtrTy, Offset, DL);
auto *Func = dyn_cast_or_null<Function>(C);
if (!Func || Func->isDeclaration() ||
Func->getInstructionCount() > FunctionSizeThreshold)
return std::nullopt;
JumpTable.Funcs.push_back(Func);
}
return JumpTable;
}
static BasicBlock *expandToSwitch(CallBase *CB, const JumpTableTy &JT,
DomTreeUpdater &DTU,
OptimizationRemarkEmitter &ORE) {
const bool IsVoid = CB->getType() == Type::getVoidTy(CB->getContext());
SmallVector<DominatorTree::UpdateType, 8> DTUpdates;
BasicBlock *BB = CB->getParent();
BasicBlock *Tail = SplitBlock(BB, CB, &DTU, nullptr, nullptr,
BB->getName() + Twine(".tail"));
DTUpdates.push_back({DominatorTree::Delete, BB, Tail});
BB->getTerminator()->eraseFromParent();
Function &F = *BB->getParent();
BasicBlock *BBUnreachable = BasicBlock::Create(
F.getContext(), "default.switch.case.unreachable", &F, Tail);
IRBuilder<> BuilderUnreachable(BBUnreachable);
BuilderUnreachable.CreateUnreachable();
IRBuilder<> Builder(BB);
SwitchInst *Switch = Builder.CreateSwitch(JT.Index, BBUnreachable);
DTUpdates.push_back({DominatorTree::Insert, BB, BBUnreachable});
IRBuilder<> BuilderTail(CB);
PHINode *PHI =
IsVoid ? nullptr : BuilderTail.CreatePHI(CB->getType(), JT.Funcs.size());
for (auto [Index, Func] : llvm::enumerate(JT.Funcs)) {
BasicBlock *B = BasicBlock::Create(Func->getContext(),
"call." + Twine(Index), &F, Tail);
DTUpdates.push_back({DominatorTree::Insert, BB, B});
DTUpdates.push_back({DominatorTree::Insert, B, Tail});
CallBase *Call = cast<CallBase>(CB->clone());
Call->setCalledFunction(Func);
Call->insertInto(B, B->end());
Switch->addCase(
cast<ConstantInt>(ConstantInt::get(JT.Index->getType(), Index)), B);
BranchInst::Create(Tail, B);
if (PHI)
PHI->addIncoming(Call, B);
}
DTU.applyUpdates(DTUpdates);
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "ReplacedJumpTableWithSwitch", CB)
<< "expanded indirect call into switch";
});
if (PHI)
CB->replaceAllUsesWith(PHI);
CB->eraseFromParent();
return Tail;
}
PreservedAnalyses JumpTableToSwitchPass::run(Function &F,
FunctionAnalysisManager &AM) {
OptimizationRemarkEmitter &ORE =
AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
DominatorTree *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
PostDominatorTree *PDT = AM.getCachedResult<PostDominatorTreeAnalysis>(F);
DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
bool Changed = false;
for (BasicBlock &BB : make_early_inc_range(F)) {
BasicBlock *CurrentBB = &BB;
while (CurrentBB) {
BasicBlock *SplittedOutTail = nullptr;
for (Instruction &I : make_early_inc_range(*CurrentBB)) {
auto *Call = dyn_cast<CallInst>(&I);
if (!Call || Call->getCalledFunction() || Call->isMustTailCall())
continue;
auto *L = dyn_cast<LoadInst>(Call->getCalledOperand());
// Skip atomic or volatile loads.
if (!L || !L->isSimple())
continue;
auto *GEP = dyn_cast<GetElementPtrInst>(L->getPointerOperand());
if (!GEP)
continue;
auto *PtrTy = dyn_cast<PointerType>(L->getType());
assert(PtrTy && "call operand must be a pointer");
std::optional<JumpTableTy> JumpTable = parseJumpTable(GEP, PtrTy);
if (!JumpTable)
continue;
SplittedOutTail = expandToSwitch(Call, *JumpTable, DTU, ORE);
Changed = true;
break;
}
CurrentBB = SplittedOutTail ? SplittedOutTail : nullptr;
}
}
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
if (DT)
PA.preserve<DominatorTreeAnalysis>();
if (PDT)
PA.preserve<PostDominatorTreeAnalysis>();
return PA;
}
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